1. Field of the Invention
This invention relates to a method and apparatus for radio or wireless communication among multiple users. It is especially suitable for use in mobile communication systems wherein a single base station must simultaneously maintain communications with a plurality of mobile stations. Cellular radio is a prime example of such a communication system. The invention facilitates error-free communication over channels characterized by fading.
In addition to mobile radio communication systems, personal wireless systems, indoor wireless networks, and digital audio and television broadcasting systems are growing in importance. However, the rapidly escalating demand for such services and the increasingly-sophisticated nature thereof have put great pressure on the limited available bandwidth within the radio spectrum. Recognizing the constraints imposed by the limited available frequency spectrum, it is dear that the most efficient possible use must be made of the available frequency spectrum in order to achieve error-free transmission and reception of signals which are primarily processed in the discrete domain.
2. Description of the Prior Art
Especially in communication systems such as mobile radio, the channels between the transmitters and receivers are likely to have a multipath nature. Consequently, the channels are characterized by fading as well as by additive noise. Although the waveform which is actually transmitted over the channel is generally analog in nature, much of the signal processing within the transmitters and receivers can be accomplished in the discrete domain. Such digital signal processing enables certain coding and decoding steps to be taken in the transmitters and receivers respectively in order to minimize errors which would otherwise occur as a result of fading and noise in the channels. Various approaches have been taken in the combat against errors by means of coding and decoding in the transmitters and receivers respectively. One such approach is employed in code-division multiple-access ("CDMA") systems. A brief description of such systems appears on pages 821-823 of Digital Communications, a book by John G. Proakis, the second edition of which was published by McGraw-Hill Book Company in 1989. On those pages, Proakis incorporates into a CDMA system a disclosure which was made on pages 800-807 of the same reference. That disclosure relates to a communication system including a modulator which impresses onto the stream of symbols being prepared for transmission a binary-valued sequence which Proakis calls "pseudo-noise." The pseudo-noise is generated within the transmitter by a pattern generator especially designed for that purpose. The receiver in the system includes a demodulator in which pseudo-noise from a similar pattern generator, synchronized with the generator in the transmitter, is employed to separate the desired estimated signal from the pseudo-noise. The separated signal then proceeds through a channel decoder and is converted into the form desired at the output. Proakis does not specify the nature of the pseudo-noise other than to say that it is a binary-valued sequence. He says nothing whatever about the Length of the sequence relative to the interval between the symbols of the data stream.